[betterprojects]

Downvoting gets unlocked at 501 Karma. Short of that, you have the ability to flag submissions where it makes sense to do so.

[dgellow]

For submissions? I have way more than 501 karma and cannot downvote submissions.

[ColinWright]

Downvoting comments and replies gets unlocked at karma 501

Submissions can never be downvoted.

Both submissions and comments/replies can be flagged, though I'm not sure if there's a karma limit on that facility.